“We had high hopes that we’d see some efficacy but we did not,” said Dr. Joseph Quinn, an author of the $10 million study and a researcher at Oregon Health and Science University.

The results with pills containing DHA, an omega-3 fatty acid, highlight “the continued frustration over lack of effective interventions” for the memory-robbing disease, an editorial said, published with the study in Wednesday’s Journal of the American Medical Association.

DHA occurs naturally in the brain and is found in reduced amounts in people with Alzheimer’s disease.

Some smaller, less rigorous studies suggested that mental decline could be slowed or prevented by eating fish, the main dietary source for omega-3 fatty acids, or supplements like fish oil pills that contain fatty acids including DHA. The study used capsules of DHA oil derived from algae.

Omega-3 fatty acids in fish or supplements have been shown to help protect against heart disease and are being studied for possible effects on a range of other illnesses including cancer and depression.

The new research involved nearly 300 men and women aged 76 on average with mild to moderate Alzheimer’s disease. They were randomly assigned to take either DHA pills or dummy pills daily for 18 months.

Results were similar in both groups; DHA provided no benefits in slowing Alzheimer’s symptoms. The pills also didn’t work even in a subgroup of participants with the mildest Alzheimer’s symptoms.

“There is no basis for recommending DHA supplementation for patients with Alzheimer disease,” the authors concluded.

Given evidence that the underlying process that causes Alzheimer’s begins years, if not decades, before diagnosis, starting treatment after symptoms appear may be too late, said editorial author Dr. Kristine Yaffe, a dementia researcher at University of California at San Francisco.

The National Institute on Aging paid for most of the research. The rest came from Martek Biosciences, maker of the DHA pills used in the study. Two co-authors are Martek employees and Quinn is an unpaid consultant to the company. Quinn and two other study authors are also inventors of a patent for using DHA pills to treat Alzheimer with a certain genetic variation.

Laurie Ryan, program director of Alzheimer’s studies at the Institute on Aging, called the results discouraging. But she noted that the institute is spending millions of dollars on research into other possible treatments including lifestyle changes, drugs and biomarkers that might lead to more targeted drug treatment.

William Thies, scientific director of the Alzheimer’s Association, said the results fit with new recommendations advocating starting treatment in the disease’s earliest stages.

“It seems clear that either we have to have more powerful drugs or they have to be used earlier in the course of the disease,” Thies said.